Evaluation of Contemporary Avian-Reptilian Phylogeny

Birds are currently defined as archosaurs, a clade of reptiles including crocodilians and dinosaurs. Birds and non-avian reptiles share enough homologous adaptations to infer birds belong with the greater clade reptiles. Such adaptations include laying of eggs outside of aqueous environment, eggs with at least some calcification, and keratinized integument with the appearance of scales. Before the emergence of recent developments suggesting otherwise, Archaeopteryx was considered to be the common ancestral species of all birds because of its bird-like morphology and feathers. Analysis of newer Archaeopteryx-like specimens from China like Xiaotingia zhengi have served as evidence to one emerging hypothesis proposing archaeopterygids belong to a sister taxon to birds. Analysis of the four-winged, arboreal Scanosoriopteryx has been used to support a hypothesis going even further to discredit the dinosaur-bird hypothesis which suggests birds have evolved from theropod dinosaurs. This research study assesses the theory that birds are derived from reptiles before proceeding to investigate the current understanding of the evolution of birds from other archosaurs.